In nuclear magnetic resonance (NMR) diagnostic apparatus, NMR signals are amplified and divided for performing quadrature detection with one phase component as a reference (real part) and the other phase component shifted by a phase of 90.degree. (imaginary part). These two signals are amplified again, analog-to-digital (A/D) converted after passing a low-pass filter, and applied to a computer in which complex Fourier transformation is performed for image reconstruction.
In the above case, if low frequency noise such as an A.C. hum is mixed on top of the NMR signal, artifacts appear at the central portion of the reconstruction image when performing image reconstruction.
We now will explain the problem described above in more detail exemplifying the concrete constuction of the conventional apparatus.
In FIG. 1, the NMR signals 5 are divided and applied to phase demodulators 1A and 1B, respectively, which phase demodulates them with reference signals r(0) and r(90) that differ in phase by 90.degree.. Demodulated signals x and y are outputted respectively from demodulators 1A and 1B. The reference signals r(0) and r(90) are generated from a 90.degree. phase shifter 3 responsive to the output from reference oscillator 2 which is commonly employed with the circuit for generating the excitation pulses, their frequency being the same as the excitation signal. The signals x and y are respectively applied to analog-to-digital (A/D) converters 6A and 6B through amplifiers 4A and 4B, and low-pass filters 5A and 5B. The outputs of A/D converters 6A and 6B are applied to a computer 7.
Assuming .alpha. is the gain for the signal system comprising the amplifier 4A, low-pass filter 5A and A/D converter 6A, and .beta. is the gain for the signal system comprising the amplifier 4B, low-pass filter 5B and A/D converter 6B, the output signals of A/D converters 6A and 6B would be represented by .alpha.x and .beta.y, respectively. After substituting .alpha.x and .beta.y as the real part and imaginary part of the signal, that signal is complex Fourier transformed in the computer 7 to obtain data in the frequency domain.
In such an apparatus if cyclic low frequency noise such as hum caused by the A.C. supply are mixed in the signal prior to phase demodulation ring-shaped artifacts appear at the central portion of the image when performing image reconstruction after complex Fourier transformation.
However, it is difficult to completely prevent the mixture of such A.C. supply hum. It is not possible to eliminate low frequency noise by filtering means such as the low pass filters 5A and 5B since they must pass the frequency band of the demodulation output signals. Accordingly, the above-mentioned artifacts die to noise prevent improvement in image quality in the NMR diagnostic apparatus.